For some applications, such as for instance maximum power point tracking solar charge controllers, DC-DC converters are required which are able to cope with a variable input voltage which, depending on conditions, may vary between greater or lower than the desired output voltage.
A commonly-used converter for such an application is a buck-boost converter. A buck-boost converter is a type of DC-DC converter that has an output voltage magnitude that can be set to be either greater than or less than the input voltage magnitude. It is a switched mode power supply (SMPS) with a similar circuit topology to the boost converter and the buck converter. The output voltage is adjustable typically based on the duty cycle of the switching transistor.
A buck-boost converter configuration is shown in FIG. 1a, along-side the conduction circuits in its on-state at FIG. 1b and in its off-state at FIG. 1c. The circuit has an input, with voltage Vin, which is switchably connectable by means of a switch S to an inductor L. Connected in parallel with the inductor L is a series combination of a diode D and a capacitor C. The output, with voltage Vout, is across the capacitor C, and load, such as a resistor R as shown may be connected across the output.
The converter operates by pulse wave modulation (PWM) as follows: while in the on-state, the input voltage source is directly connected to the inductor (L). This results in accumulating energy in L. In this state, the capacitor supplies energy to the output load, both as shown schematically in FIG. 1b. 
While in the off-state, the inductor is connected to the output load and capacitor, so energy is transferred from L to C and R, as shown schematic in FIG. 1c. 
In the most widely used, continuous conduction, mode (CCM), it can be easily proved based on the energy equation in steady state that the duty cycle of the PWM can we written asD=Vout/(Vout+Vin).Thus the circuit has to operate with a duty cycle less than 50% in the buck mode and greater than 50% in boost mode of operation.
It should be mentioned that one possible drawback of this converter is that the switch does not have a terminal at ground; this complicates the driving circuitry. Another is that the polarity of the output voltage is opposite that of the input voltage. Neither drawback is of any consequence if the power source is isolated from the load circuit (if, for example, the source is a battery, such as is the case for a solar charge controller) as the source and diode can simply be reversed and the switch moved to the ground.
In general, a buck-boost converter, operating in buck mode, is less efficient than a buck converter. The main reason for this is that, for a buck-boost converter, the inductor current flows to ground during the ON time and only a fraction flows to the output during the OFF time, especially in continuous mode of operation. In contrast, in normal buck converter operation the current is transferred to the load through the inductor in the ON cycle, whilst out of this a small portion is stored in the inductor. This stored current is then pumped into the load during the off cycle. In applications where the charger operates predominately in buck mode, it is important to improve the efficiency of the circuit in buck mode of operation.
A DC-DC converter which is switchable between a boost-mode and a buck-mode is known, for instance as shown in FIG. 2. This shows a cascaded buck-boost arrangement which is similar to the conventional buck-boost arrangement, except that instead of an inductor, there is an input diode (D21) switchably connected across the input; the inductor is in series with the output diode (D22) and capacitor (C), and there is a second switch S22 connected between the output side of the inductor and ground.
However, this circuit suffers from the disadvantages that in the buck mode of operation the free wheeling current has to go through 2 diodes (D22 and D21) resulting an additional power drop, which may be significant for high current levels. Furthermore, both the switches S21 and S22 are switching elements and need gate drive circuits to be added to the overall design which consumes power. There thus results in extra power losses occurring due to these switching elements. Furthermore, a larger number of components needed. Finally, the arrangement is a synchronous converter which is based on synchronous switching of the circuit elements, and thus requires careful design to avoid overlap or dead zones of conduction while making the transitions.
There is thus an ongoing need to provide a DC-DC converter, which can operate as a buck boost converter, which suffers to a lesser extent from some all of the above disadvantages.